As a geothermal field ages by reason of long term extraction of hot geothermal fluid from geological formations in the field, and long term reinjection of spent heat depleted geothermal fluid into the field, two results often occur: 1) a reduction in the mass flow rate and temperature of hot geothermal fluid from production wells in the field; and 2) an increase in the pressure required to reinject heat depleted, waste or disposable geothermal fluid (i.e., spent geothermal fluid) into injection wells in the field. Both problems apparently result from reduction in the permeability of the geological formations in the field.
Another common problem that often occurs in an operating geothermal field is coupling between an injection well and a production well whereby the heat depleted geothermal fluid pumped into the injection well begins to adversely affect the temperature of the hot geothermal fluid extracted from a nearby production well. The exact mechanism that controls these results is not entirely understood at this time, but it may be that the problems are related to the permeability, or changes in the permeability, of the geological formations in which the wells are located. Such changes apparently are caused by long term extraction of hot geothermal fluid from, and long-term injection of spent geothermal fluid into, the geological formations of a field.
In order to increase the rate at which spent geothermal fluid can be disposed of in an injection well, it is conventional to carry out what is termed "acidizing" the well. Such an approach is also used for increasing the flow rate of geothermal fluid from a production well. This is a procedure by which concentrated acid is applied to a stratum whose location and nature is determined from geological studies for the purpose of increasing the permeability of the stratum by the dissolving action of the acid on the rock in the stratum. The procedure, which is time consuming, costly, and dangerous, requires the injection well to be taken out of operation.
The first step after operation of the injection well is terminated is to place rubber seals in the well at the upper and lower limits of the stratum to be treated. These placements require the use of a rig having drilling pipes for inserting and withdrawing the rubber seals; and usually, a considerable investment in time and labor is involved. Next, a pipe is run down the well from the surface to the upper seal for gaining access to the region of the well between the rubber seals. An acid solution of 10-20% concentration is then pumped through the pipe into the volume between the seals for a period of from 1 to a few hours. Both water and acid must be trucked to the well-head for this purpose; and a considerable volume of liquid is required for this operation. The acid solution percolates through the stratum defined by the seals and dissolves some of the rock therein increasing the permeability of the stratum. Finally, the rubber seals are removed by the rig, and operation can be resumed.
This is a batch approach to increasing the permeability of a stratum. It is expensive because the well being treated must be taken out of operation and testing of the efficacy of the treatment can be done only by closing the well and resuming operations. This results in a tendency to over-treat a well which further increases the cost in money and time.
Recently, geothermal engineers have developed an alternative to the batch approach. A carrier fluid, such as water or a spent geothermal fluid, has been continuously applied to a geothermal well, using (for example) a pump, and concentrated acid has been added to the carrier fluid over days or a few weeks, rather than hours as in the case of a batch process. The concentration of acid in such carrier fluids has been from 0.1% to 10%, which still effects a rather acidic pH of less than 1.55 and can still corrode much of the well hardware. In most instances, corrosion resistant material, e.g., titanium, is inserted into the geothermal well or utilized above ground.
Presently, a need exists to provide a new and improved method of and means for operating and acidizing geothermal wells to selectively modify the permeability of geological formations in which both injection and production wells are located.